Saturday, May 21, 2011

Japan's Radioactive Tea

A type of tea in Japan has been found to be contaminated with Cesium 137. The Japanese governments limit for radiation of the tea is 500 Becquerels per kilogram. Since tea is pretty important in Japanese culture, some citizens are wondering if the limit is really needed. The Japan Probe post on Radioactive Tea in Japan gives a good account of the situation. One of the main issues is tea is dried, steeped then drank, but the Japanese regulation is for un-dried tea like it is a vegetable to be eaten.

The unit Becquerel means one disintegration per second. In the USA we use the unit Curries were one Currie is equal to 37 million disintegrations per second. A disintegration is a pop or count on a Geiger counter. For the average tea drinker this is pretty scary and most have know clue it the tea is really bad for them since they have know clue what a Becquerel means health wise.

The Nuclear Regulatory Commission in the United States came up with the Banana Dose to help people understand radiation. The Banana Dose (BD) is considered to be 520 picocurries. Using a handy dandy converter a BD is 19 Becquerels. So 500 Bequerels is a bunch of bananas. Since most people don't eat tea fresh, it is dried first which decreases its weight which would increase the radiation per weight. But only a small amount of dried tea leaves are used at a time and not all the radiation may be dissolved in the tea after steeping. Interesting puzzle.

After drying, tea weighs about 15% of what it did straight off the tree. So a kilogram of dried tea that tested at 500 becquerel per kilogram fresh, would test roughly 3333 becquerel per kilogram dried. The average strong tea bag contains roughly 3.3 grams per Wikipedia, so a mug of radioactive tea would contain about 11 Becquerels. What I am calling a mug is about 1/4 liter. So one liter of radioactive tea would be about 44 Becquerels worth of radiation, if all of the radiation in the tea leaves dissolved in the water. For infants, the Japanese limit per liter of water for radiation is 100 becquerels and adults 300. So at first blush, it does not look like the 500 Becquerel limit for fresh tea leaves is all that reasonable. One thousand Becquerels per kilogram of fresh tea would bring the per liter of consumed tea to 88 Becquerels, which should be safe even for infants if it were water, milk, juices or any other liquid.

I just used rough numbers, but it looks like the tea should be pretty safe. Since the 100 Becquerel limit is very conservative to begin with, I am pretty sure it is safe at 1000 Becquerels per kilogram of fresh tea leaves. I would think the Japanese government would test the tea as it is used before setting a weird limit on a product unless there is a common food use for fresh tea leaves. It would also be nice if they used the banana dose equivalent more when describing the safety of food stuffs.

It would also be interesting to know if vegetables were tested after rinsing. Most radioactive isotopes wash into the soil. Some are taken in by growing plants, but the majority remains in the soil. Cesium is absorb in the human body similarly to potassium. According some of the sources I have read, most of the cesium ingested passes through the body in about 100 days. Since it is chemically similar to potassium and sodium, that makes sense, but more could be retained.

While Fukushima is a dangerous mess, there is plenty to be learned from the misfortune of the Japanese people. Hopefully, a better understanding of ionizing radiation will be one of the lessons learned.

1 comment:

Alex Zorach said...

Geiger counters aren't terribly expensive; someone in Japan could test this, if they could get their hands on some of this radioactive tea. This would be interesting...and it certainly would provide useful information.